This invention relates to the preparation of a honeycomb structure having at least some channels that are sealed at one of the faces of the honeycomb and to the use of such a honeycomb for separation or concentration of oxygen. More particularly, the invention relates to honeycombs made of ceramic material of a solid electrolyte nature having channel walls that are permeable to oxygen ions. Such structures are particularly useful for the concentration of oxygen from air in small scale "point of use" systems for the production of oxygen.
For example, medical grade oxygen for health care, including hospital and home care, has been supplied by three primary sources--cryogenic, compressed gas tanks, and oxygen concentrators. Although all the sources are suitable for hospital use, where size and patient mobility are not necessarily of concern, none has been entirely satisfactory for home health care, where efficiency and mobility of the source for the patient and for the therapist become more important. For example, cryogenic sources are too large and cost ineffective for low volume home care, and compressed tank oxygen, although more portable and efficient, requires constant replenishment. Currently available oxygen concentrators, which have been used for home care as well as hospital care, can produce as much as three liters of 90% oxygen through the use of zeolite sieves to separate oxygen from ambient air. Although not requiring constant replenishment, these concentrators, at approximately 80 pounds, are still not easily portable.
In order to provide a concentrator of much smaller size, the present invention takes advantage of the high surface area and solid electrolyte properties of certain ceramic materials in a separation system in which oxygen ions are transported through the walls of a ceramic honeycomb by a difference in electrical potential between adjacent channels. It is known that certain electrically conductive, oxygen-permeable materials can be used as solid electrolyte separator plates or membranes in a process in which oxygen is driven from one side of the plate to the other by the application of a direct voltage across the plate. See, for example, U.S. Pat. No. Re 28,792, which discloses an electrochemical device useful as a fuel cell or oxygen separator/concentrator based on the use of plates of certain ionically conductive oxides as the solid electrolyte through which the oxygen ions are passed. U.S. Pat. No. 4,175,165 discloses another fuel cell assembly involving a multitude of solid electrolyte plates that are placed in parallel and separated by an alternating series of electrically conductive bipolar plates. Although these designs are useful generally in the operation of fuel cells or oxygen separators/concentrators, they have practical drawbacks in that materials used to seal the plates must be able to withstand operating temperatures of about 800.degree. C. without cracking. Moreover, when a group of cells is to be used, they must be stacked and wired in series, so that a failure of one cell causes failure of the entire stack.
In order to overcome these and other problems, the present invention replaces the complicated structures of the prior art by a relatively simple monolithic ceramic honeycomb of particular composition (preferably stabilized zirconia, .beta.-alumina, thoria, and/or mullite) having inlet channels and outlet channels that are spatially arranged and electrically connected so as to emulate a series of stacked flat plates. In such a system, the entire cell can be of a single material, which eliminates many cracking problems caused by the use of different structural materials having different thermal expansion characteristics, and provides to the cell the mechanical strength and thermal stability that ceramic materials are known to have.